Permeation grouting is typically used to improve the structural characteristics of the soil, as well as several secondary uses such as reducing soil permeability, improving soil cohesion, or, a combination of such. Permeation grouting is sometimes performed by injecting any of a variety of grouts into the soil to permeate into void spaces and connect individual soil grains without otherwise disturbing the natural state of the soil.
Chemical-permeation grouts have been used for decades in applications ranging from water stops to soil densification. The behavior of chemical-permeation grouts, once injected into a soil medium, is difficult to model and understand. The injection of chemical-permeation grouts was once promoted as “BLACK MAGIC” that only certain contractors were capable of doing, but is now a respectable branch of the ground modification arsenal available to today's Civil Engineers and geotechnical contractors. Chemical grouting was introduced as a means to treat the soil profiles that could not be addressed by compaction or intrusion grouting. Prior to the introduction of chemical grouting for soil stabilization, contractors were limited to slurry grouting. Slurry grouting is a type of permeation grouting using the injection of cement grout of a high slump under lower pressure in an attempt to permeate the void spaces between soil grains and bind the soil matrix together. Slurry grouting is a very messy process. Additionally, achieving permeation of the void spaces between soil grains with slurry grout had limitations due to the nature of the grout material properties. Slurry grout is a suspension of cement particles that must flow into the void spaces between the soil grains in order to be effective. The interaction of the suspended particles and the soil grains restricts the amount of grout that will permeate effectively.
A more efficient solution is to introduce a chemical grout comprised of low-viscosity fluid instead of a suspension of particles to increase the magnitude of permeation and allow a larger area of influence at each injection location. Although permeation grouting with chemical-permeation grouts has been performed in the past, the existing delivery systems are poorly suited to the application in various soils. The common method for the injection of chemical-permeation grout utilizes an airless paint spray pump attached to an injection pipe with a single opening below grade at the injection depth. The only controls available for injection are basically “ON/OFF”. Under these equipment limitations, there is no accountability, no recordable data except estimates of injected volumes and no ability for the Engineer of Record to adjust the injection protocol to achieve site-specific goals. Some available models of airless paint sprayers have adjustable pressure outputs, but do not precisely control or measure injection pressures. As such, permeation grout is injected at pressures of up to 3000 psi, without the contractor or the Engineer having any knowledge or control of the injection pressure. The permeation grout is typically injected in small batches, usually five gallon buckets. Having an open system (open bucket) subjects the permeation grout to moisture that often leads to premature gel formation of the material in the bucket, in the sprayer, in the lines, etc. As the chemical-permeation grout gels prior to introduction into the soil mass, permeation often becomes hindered to the point that very low grout takes are recorded even in the most permeable soils. Data typically shows either very low grout takes (due to premature gel formation), very high grout takes (due to extreme injection pressures) or, worse yet, a combination of the two at a single site. Without reliable data and precise injection controls, the Engineer of Record has no mechanism to differentiate between a failed injection attempt, a very dense in-situ soil, a large subsurface void or soil that has been literally blown apart and then replaced by grout.
Approximately a decade ago, two-part polyurethane foams were introduced as a solution to some such issues. Since then, two-part polyurethane foams have gained popularity very quickly and dominated the shallow soil stabilization market.
Originally, 2-part polyurethane foam injection was developed for void filling at the soil-to-slab interface and for lifting and leveling of light structures. In these applications, 2-part polyurethane foams are far superior to any other product on the market. As the search for available soil densification techniques widened, 2-part polyurethane foams were selected for injection below grade to increase bearing capacity via displacement. The thought was: inject bulbs of rapidly expanding chemical grout at discrete locations and depths where the polyurethane foam would expand rapidly to displace loose soils and compact the soils within the immediate vicinity of the grout bulb. One such system is described in U.S. Pat. No. 6,634,831. Many residential buildings received the 2-part polyurethane injections as disclosed in this patent to stabilize loose granular soils as part of sinkhole remediation packages.
When treating soils with chemical grout, it is desired to limit further harm to any existing soil structure. The golden rule of “do not disturb the soil” is the guiding parameter. At low pressure the physical properties of the permeation grout achieves the desired soil modifications, but permeation is low. Uncontrollable, high-pressure applications are utilized in an attempt to ‘force’ permeation or induce displacement of in-situ soils. High pressure injection (in excess of 150 psi) is sometimes detrimental to the soil profile except in instances where the Engineer of Record wishes to cause soil fractures (as in the case of lens grouting or fracture grouting). The 2-part polyurethane foams (one part plastic and one part reactor) are commonly injected at pressures that induce soil fractures and cause the chemical grout to migrate away from the location being treated. Once the soil is fractured, there is no way to predict the final location of the injected grout. To exacerbate the problem, once the rapidly expanding foam product is injected into a formed fracture, the expansion of the product pushes the fracture to spread to even greater distances and then forces the fracture apart. The use of 2-part polyurethane foams for densification and increased bearing capacity is flawed in the same aspects that the early delivery systems for permeation grouting were flawed.
In order to achieve the desired modifications in, for example, a granular soil profile, a new process is needed.